Calcium Solubility, Intestinal Sojourn Time and Paracellular Permeability Codetermine Passive Calcium Absorption in Rats

Abstract

To investigate the nonsaturable, paracellular pathway of intestinal Ca absorption, the luminal contents of 12-cm segments of the intestine of 8-wk-old male Sprague-Dawley rats were analyzed for pH, sojourn time and soluble and insoluble Ca over a 24-h period. The rats had been fed one of two high Ca diets for 2 wk: 1.5% Ca (diet group 3a) and 3.1% (diet group 5a). The pH of the small intestine increased from < 6.6 to > 8.0 from duodenum to ileum; transit time increased from 2.5 min in the duodenum to 58 min in the distal ileum, with the entire ileum accounting on the average for 74% of the transit time of 3 h. The amount of Ca solubilized throughout the intestine was 32 +/- 3.3 mumol in diet group 3a and 53 +/- 5.3 mumol in diet group 5a, i.e., 2.7% and 2.0% of the total luminal Ca. Because absorption by diet group 3a was 1.45 +/- 0.23 mmol/d and that by diet group 5a was 2.50 +/- 0.18 mmol/d, the amounts absorbed were 45.3 and 47.1 times greater than present in the lumen in soluble form at any one time. Thus, over a 24-h period, an average of 3.2% (46.2/1440) of the soluble Ca present in the lumen at any time was absorbed per min. Calculations involving the gradient between luminal and plasma Ca show that the rate of Ca diffusion from lumen to blood is < 2% of what it would be if the paracellular path were unrestricted. Thus, intestinal sojourn time, Ca solubility and mucosal permeability to Ca are factors that determine the rate of passive Ca absorption.

Difructose Anhydrides III and IV Equally Promote Calcium Absorption from the Luminally Perfused Rat Small Intestine

“…2). These results agree with the findings of the in vitro experiments with isolated mucosa of the small intestinal, 7,10) and are concrete and direct evidence for enhancement of Ca absorption by DFAs in the small intestine.…”

“…Previously, we found that DFAIII promotes Ca absorption by activating the paracellular transport pathway through tight junction. 10) It has been reported that transit speed in the small intestine is an important factor for determination of diffusional Ca transport via the paracellular pathway, especially at high Ca levels in the diet. 11) We used a perfusion medium with a high but physiological Ca concentration, 10 mmol/l, 12) much higher than the Kt of active Ca transport, 0.32 mmol/l, in the duodenum.…”

Chymotrypsin C (caldecrin) promotes degradation of human cationic trypsin: Identity with Rinderknecht's enzyme Y

“…Although intestinal Ca 2ϩ absorption has been studied extensively, reliable data on the ionized Ca 2ϩ concentrations along the small intestines is lacking (28). It is noteworthy that ionized Ca 2ϩ concentrations in the gut are largely determined by luminal pH and insoluble complex formation, which becomes more significant at the more alkaline pH of the lower intestines, where trypsin degradation was shown to occur (29).…”

Mannitol enhances intracellular calcium diffusion in the rat ileum - a hypothesis

“…The reason for this is the activation of the Na + /H + exchanger by a lower Na + concentration, as demonstrated with the aid of inhibitor studies [20]. It is unlikely that the drop in pH caused transcellular calcium transport, inasmuch as addition of mannitol or other polyols [15,28] to jejunal loops which are more acidic (pH < 6.6) [11] had no effect on transcellular calcium transport.…”

“…In the absence of calbindin D 9k , self-diffusion of the calcium ion is so slow that the transcellular component is essentially nil and absorption is only mediated by the paracellular process, dominant in both jejunum and ileum. However, because the time spent by chyme in the ileum accounts for three-fourths of the 3 h that the chyme sojourns in the small intestine, the ileum is the principal site of calcium absorption [11,25].…”